Chemistry Reference
In-Depth Information
with F
p
being the quantum yield of phosphorescence in the presence of quencher
and
Φ
0
is the phosphorescence yield in the absence of quencher. Stern- Volmer
analysis consists of plotting
p
as a function of [DNA] times the ES lifetime t
0
in the absence of quencher, providing a linear relationship with a slope equal to
k
SV
,
where
k
SV
=
k
et
+
k
q
. In the absence of measurable ES emission,
k
et
may be deter-
mined by observing the changing in ES lifetime as measured by transient absorption
spectroscopy because:
ΦΦ
p
0
ΦΦ
p
0
= ττ
0
(8.23)
p
where t is the excited state lifetime in the presence of quencher. Transient spectros-
copy can differentiate electron transfer quenching and other decay pathways probing
the formation of the electron transfer product. Measuring the rate of formation of
the product can also allow for the determination of
k
et
.
Excited State Energy Transfer Theory
Two mechanisms of excited state energy transfer (Equation 8.9) have been described
and involve either ES dipole resonance coupling or electron exchange between the
*LA donor and quencher, Q (Equation 8.9). These two mechanisms have been
described individually by Fö rster
47
and Dexter.
48
F örster described *LA quenching by excited state energy transfer through
space by a Q.
47
Resonant coupling of the chromophore and quencher excited state
dipoles allow for radiationless energy transfer from the *LA donor to the acceptor
Q. Förster energy transfer effi ciency is governed by the spectral overlap of *LA
emission and Q absorption, interchromophore distance and relative dipole orienta-
tion. Förster excited state energy transfer is the dominant pathway when *LA
→
LA
and Q
* Q transitions are spin allowed. F örster excited state energy transfer is
the governing principle of fl uorescence resonance energy transfer (FRET) tech-
niques used in confocal microscopy.
Another mechanism of excited state energy transfer, described by Dexter,
involves electron exchange between the *LA and Q.
48
Electron exchange requires
some electronic coupling of the *LA donor and the Q acceptor orbitals. Dexter
transfer is not bound by the spin selection rule and is thought to be the dominant
mechanism of *LA energy transfer when there is poor spectral overlap of *LA
emission and Q absorption, e.g. photosensitization of
3
O
2
→
1
O
2
.
Study of excited state energy transfer quenching dynamics can be performed
using Stern-Volmer kinetic analysis. Details of this analysis are outlined above for
excited state electron transfer. In some instances, observation of the rise-time and/or
decay of the *Q state can also be observed and aids in understanding this bimolecu-
lar interaction for a specifi c system.
→
Type II Photooxidation Reactions in PDT
Type II photooxidation reactions of biomolecules, as defi ned by Foote,
46
involves
photosensitization of molecular oxygen (
3
O
2
), generating singlet oxygen (
1
O
2
) which
reacts with a biologically important substrate molecule. Photosensitization of
1
O
2
